Wave translating system



' lations generated in the Registered June 17, 1 941 T OFFICE i T WAVETRANSLATlNG SYSTEM ,ns mia s; Black, Elmhurst, N. Y., assignor to Bell II Telephone Laboratories,

Incorporated, New

j York, N. Y., a corporation of New York Application May 27, 1938,Serial No; 210,333

' 11 Claims. 01. 179-171) This invention relates to wave translation, asfor example, electric wave amplification, and relates especiallyto'retroactiveor feedback systems, as for example, negativefeedbackamplifiers. J t

Objects of this invention-are to control feedback and singing orproduction of self-sustained oscillations in such systems; a

It is also an object to increase the amount of useful non-oscillatoryfeedback obtainable in such systems. as for example, the amount ofnegative feedback of utilized waves obtainable without "unduedegradation of the performance of the systems by incidental' oscillatoryfeedback ofunwanted waves. J

In one specific aspect the 1 invention is .embodied in a broadband-vacuum tube amplifier with'large negativeffeedback. In this type ofcircuit, and in many other types 'of feedback circuits, prevention-ofsinging is a'major problem, but it is often relatively easy to-insurethat the frequency at which the circuit does singisconsiderablybeyond'or' outside-the used frequency band. For example, in'a'12-60kilocycle ampli-v fier the singing frequency may be about 1,000,000

cycles. r

Ordinarily the chief objection-to singing is that the tubes overload andhence the modulation is too great and in other respects the transmissionof the amplifier is impaired. 1 However,

as pointed out in R. S: Caruthers Patent 2,066,333, January 5, l937,-theamplitude'of sustained oscillations in an oscillator circuitcan belimited toa value well below theoverload point of the amplifyingelement, by an amplitude limiting device other 'than'the amplifyingelement, and then the amplifying element can be used for amplifying notonly the sustained osciloscillator. circuit but also other waves. V 4

.Therefore, in the above-mentioned specific aspect'of the presentinvention, in accordance with ter, as for example, the Thyrite, copperoxide the invention,,.the amplifier is allowed to oscillate or singaround its negative feedback loop outside the used frequency bandv(since satisfying the requirements for preventing oscillation would beso difficult), but the amplitude of the unwanted oscillationscirculating around that loop is limited to a value sufliciently 'smallto avoid overloading the amplifier or reducing the gain of itsamplifyingelement, for example, a value so minute as to avoid appreciably reducingthe useful output capacity of the amplifier even though it is thusoscillating. Further, the element that limits the amplitude of theoscillations is so' associated with that loop, for example, through aband suppression filter whose attenuation range is the used frequencyband, as to be not acted upon or not appreciably affected by theutilized waves or waves, in the used frequency band that circulatearound that loop. Therefore the amplitude limiting element does notprevent or interfere with the large negative feedback around that loopin that band, .(Although, for any greatly exceed thegamplitu'de of theoscillations,

or on the other handfmay, for instance where the amplifierlis 'used formeasuring purposes, be small (for example even smaller than theamplitude of the oscillations), with the negative feedback neverthelesslarge.

The amplitude limiting means or. element may be a non-linear elementof-any suitable characrectifier, or space discharge device referred tointhe above-mentioned Caruthers patent as exhibiting an overload ornon-linear relation between current and voltage, or' any suitabletemperature dependent resistance of the general type that varies itsresistance greatly in response to change of its temperature. Examples ofsuch temperature-dependent resistors or so-called thermistors, whichchange the magnitude of their resistance enormously in response totemperature' change, even when only heated by the current passingthrough them,.are resistors of silver sulphide, uranium oxide, cobaltoxide or boron. The term non-linear resistance is used herein in thebroad sense of resistance having a characteristic that isnon-linear, andincludes resistances, such for example as these thermistors, whichnevertheless are linear in the sense that they introduce no materialamount of deleteriousmodulatiori in the useful frequency range, becausethe time required to alter their temperature ;and,hence, resistance, isappreciable as compared to the more rapid changes in instantaneouscurrent or voltage at any frequency falling within the useful band offrequencies of the amplifier. 5 5

The amplitude limiting element may be anywhere in the negative feedbackloop. However, ordinarily it preferably is in the forward or tageousbecause the smaller voltage at which limiting is desired the lessreadily the limiting isv accomplished, in practice. If the gain of the,u-circuit is 120 decibels limiting in the output at Caruthers patent,connected in the loop. The element 4 may be, for instance, a thermistorof any of the types mentioned above with negative temperaturecoefficient of resistance, or Thyrite or a copper oxide rectifier. Asexplained in the above-mentioned C'aruthers patent, if the voltage inputto the bridge network had a certain predetermined value the. non-linearresistance would balance they-bridge andlthe voltage output of thebridge network would be zero, and therefore (as also explained in thepatent) the amplitude of the sustained oscillations at the vinput tothebridge is slightly below that pre- 1 volt would be equivalent tolimiting-at000001 volt at the input.

A feature of the invention is limitation'of'the amplitude of incidental,unwanted, self-sustained oscillations in a feedback system, as forexample an amplifier with large negative feedback, to'a value below theoverload point of the system.

Another feature is accomplishing such limitation without therebynecessitating limitation of feedback of utilized waves inthe system.

A further feature is reduction of the operating margin required againstsinging in a negative feedback system, as for example an amplifier withlarge negative feedback, or elimination of necessity for such margin.

Other objects, aspects and features of the invention will be apparentfrom the following description and claims.

Figs. 1 and 2 show two negative feedback'amplifiers of the type referredto above; and.

Fig. 1-A indicates a modification ofthe amplifier of Fig. 1.

The amplifier of. Fig. 1 may be for example, of the general type showninFigs. 1,6 and 7. of my copending application Serial..No..114,390,filed December 5, 1936, for Wave translation systems, now Patent No.2,209,955, issued August 6,1940. It comprises an amplifying path.including an amplifyin element or device having any desired number ofcascaded vacuum tube stages, G and P designating the grid of the firsttube andthe plate of the last tube (in this figure and also in Fig. 2),and comprises a feedback path I. The amplifying path may be referred .toas the l-circuit and the feedback path may be referred to as theo-circuit, the significance of and ,8, as well a [L-CiI'Cuit andB-circuit being as indicated in my above-mentioned Patent 2,102,671.

An input bridge transformer or hybrid coil 2 couples the incomingcircuit and the feedback'.

path I to the input end of the amplifying path; and an output hybridcoil ,3 couples the output end of the amplifying path to the outgoingcircuit and the feedback path. If desired, the

amount of the negative feedback (in the used frequency range), or the.gain reduction due to feedback, may be large. For example, s may be ofthe order of one or more times decibels.

The amplifier may be designed to pass and a lplify frequencies from 12to 60 kilocycles for 'g when the feedback connection is a de theamplifier may sing around the negative feedback loopat 1,000,000 cyclesfor instance. In this singing or oscillatory loop is included example,and

means for limiting the amplitude of the sustained oscillations to valuebelow the overloadpoint'of the amplifying tubes or element. This means,as shown, comprises a non-linear resistance 4 inpne arm 5 of aWheatstone bridge network 6, such as the bridge of Fig. 3 of theabove-mentioned the point of balance. of the bridge may be condeterminedcritical value.

In Fig; 1 the element 4 is preceded by a band lsuppression network orfilter 1 which attenuates or suppresses waves of the used frequencyband,

taining it, and, as explained in the, Caruthers patent, by properlyproportioning the resistance of e1ement:4 andathe-resistances.formingthe other three ratio armsof the:bri dge-, thebridge. may be sounbalanced when; the amplitude, of the waves. of oscillation frequencyis below the critical value that as the amplitude, of those wavesincreasesthe bridge approaches a condition of balance because of.therchange in resistance of thelelement 4.and-.final1y thelimitingpoint(i. e. the point where the; magnitude of. porthe feedback ratio is unityfor the oscillation frequency) is. reached as the result ,ofthetwofoldaction of the bridge and=of. the non-linear resistance of the element 4.As explained in the Caruthers patent,;by-adjusting the resistances ofthe ratio arm other than the arm containing element 4,

trolled andthus the limit ofamplitude of our- -,;rents-general ed-may'be controlled. As indicated above,, the .amplituderof the sustainedoscillations may thus be limited to a value sufficiently small to avoidoverloading the amplifier or reducing the gain of its amplifyingelement, for

example-a value even so minute as to avoid. appreciably the usefuloutputcapacity of the amplifier. As also-indicated above, the filter 1prevents the signal' Waves or waves of the used frequency band thatcirculate around the negative feedback oop from acting upon orappreciably aifecting the elementv 4 inthe same loop'.

The impedance of filter -'I viewed from the .bridge'network 6 maybe alowimpedance outside the used frequency band. Whether it is high or lowaffects eitherthe poling of the transformers or the'bridge arm in' whichit is; located. Accordingly, the filter I and amplitude limitingresistance ,4 may: preferably be in the arm. 5 of. the bridge network 6,asshown, rather. than in. the opposite arm, because when inthe arm 5they are in the i-circuit of theamplifier.

Theamplifier of Fig; 1 oscillates or sings-weakly asdescribed above'at1,000,000 cycles; and if 'there. is a lowfrequency phase cross-over asfor example, at cycles (that is, ifat a frequency below theused'bandf'as for instance a frequency .of 100 cycles, the phase shiftaround the feedback loop. is zer /and the. gain around the loop exceedsZero decibel) the amplifier will oscillate weakly atboth 1,000L000cyclesand I00 cycles at the same'time if the bridge arm containingplication of the amplifier.

the amplitude limiting element be changed, for

in which are connected two filters I and 1" each feeding a non-linearresistance 4 such as that of Fig. 1. The filter 1 is a -12 kilocyclelow-pass filter, transmitting to its associated element 4 waves offrequencies below the used frequency band and attenuating waves ofother.

frequencies. The filter 1" is a high-pass filter with cut-off at 60kilocycles, transmitting to its associated element 4 waves offrequencies above the used frequency band and attenuating waves of otherfrequencies.

The invention comprehends application in which unwanted oscillationsmay, without .01)- jection, be permitted to fall within the usedfrequency band, as well as applications in which the unwantedoscillations are outside the used band. Also, the invention comprehendsapplications in which an oscillation frequency may lie in an unusedfrequency range between two used frequency bands, as in the case of atwo-band amplifier for a single-amplifier type of repeater for amultiplex carrier telephone system.

If desired, filters may be used to connect the amplifier to its attachedincoming and outgoing circuits, for excluding the weak oscillations ofsing frequencies from those circuits. ple, filters indicated in Fig. 1as band filters 8 and 9 may be low-pass filters having cut-01ffrequencies of 60 kilocycles to serve this purpose. The filters for thispurpose may be of any suitable type, for, instance 12-60 kilocycleband-pass:

filters, or high pass filters attenuating frequencies below 12kilocycles in case the sing frequency is below instead of above the usedfrequency range of 12-60 kilocycles. Either the filter at the input orthat at the output, or both, may be omitted, depending upon the specificap- In the amplifier of Fig. 2 the band suppression filter 1 feeding thenon-linear resistance 4 is shown across the plate circuit of the lasttube and in the -circuit of the feedback am- Then the input impedance offilter I viewed from the amplifier ordinarily is preferably made highfor the used frequency band.

The amplifier may be, for instance, a 12 to 60 kilocycle negativefeedback amplifier similar to that of Fig. 1, and is shown by way ofexample with input bridge network l and output bridge equalizer networkI 3 like those of A. L. Stillwell Patent 1,993,758, March 12, 1935,in-stead'of the input and output bridge transformers 2 and 3 of Fig. 1.The equalizer bridge comprises arms designated R0, Z21, R and Z11 as inthat patent, and functions as explained therein. The amplifieroperatesin the same general manner as that For examof Fig. 1 herein, thenon-linear resistance limiting the amplitude of the oscillations of thesing frequency (for example 1,000,000 cycles) and the band suppressionfilter preventing the signal waves or waves of the used frequency bandfrom acting upon or materially affecting the nonlinear resistance, asbrought out above. If desired or necessary for modulation reasons, thefilter 1 feeding the amplitude limiting element 4 in Fig. 2 may beconnected across the -circuit of the feedback amplifier at an earlierpoint than shown. The disadvantage of picking off the oscillatingfrequency whose amplitude is to be controlled earlier in the -circuit isthat its energy level is weaker, the advantage is that the entire loopof signal waves of a given frequency range,

amplitude discriminating means for. connection to said loop to limit theamplitude of waves pass- .ing around said loop, and frequency selectivemeans for connecting said amplitude discriminating means to said loop,said frequency selective means having its attenuation. for transmission.from said loop to said amplitude discriminating means high throughoutthe signal frequency range, but low over a wide frequency rangeexclusive of the signal frequency range.

2. An amplifier comprising a forwardly transmitting path including awave amplifying device and a feedback path forming with said first patha closed. loop path producing such large negative feedback of waves ofsuch a wide frequency band to be amplified by said amplifying devicethat said loop path produces as--a consequence of the magnitude of saidnegative feedback and the width of said band self-sustainingoscillations around said loop path of a frequency above said band andsimultaneously of a frequency below said band, a non-linear resistance,a frequency selective circuit connecting said resistancein said firstpath and having high attenuation for Waves of said band and waves ofsaid frequency below said band but relatively low attenuation for wavesof said frequency above said band, a second non-linear resistance, andasecond frequency selective circuit connecting the latter resistance insaid first path and having high attenuation for waves of said band andwaves of said frequency above said band but relatively low attenuationfor waves of said frequency below said band.

3. A wave translating system comprising a source of signaling waves of awide frequency band to be amplified and a wave amplifying circuitconnected thereto for amplifying said waves, said circuit comprising anamplifying device and afeedback path forming therewith a feedback looppath having its'loop propagation over said band such as to producenegative feedback of said signaling waves around said loop path, saidloop path having its loop propagation at a given frequency such as toproduce self-sustained oscillations at said frequency, and amplitudediscriminative means non-linearly responsive to amplitude changes ofimpressed waves for limiting the amplitude of said oscillations to theexclusion of said signaling waves to a value sufficiently low to preventsaid oscillations from materially reducing the load capacity of saidsystem in amplifying said signaling waves.

4. A wave translating system comprising a source of signaling Waves of awide frequency range to be amplified and a wave amplifying circuitconnected thereto for amplifying said waves, said circuit comprising anamplifying device and a feedback path forming therewith a feedback looppath having its loop propagation over said range such as to producenegative feedback of said signaling waves around said loop path, saidloop path having its loop propagation at a plurality of frequencies suchas to produce self-sustained oscillations at all of said frequenciessimultaneously, and amplitude discriminative means non-linearlyresponsive to impressed voltage changes for limiting theamplitudes ofsaid oscillations at said frequencies'to the exclusion of said waves tovalues sufficiently low to prevent 'said' oscillations from materiallyreducing the load capacity of said system in amplifying said as toproduce incidental singing around said loop at afrequency outsidesaid'range and means to prevent said singing from rendering saidamplifier useless and thereby increase the maximum amount of saidnegative feedback that can be usefully employed as compared to thenon-singing condition, said means including a non-linear resistance forconnection in said forward path to limit the amplitude of waves of saidsinging frequency to an amplitude such that said singing is preventedfrom overloading the amplifier, and means selective to frequenciesoutside of said frequency range for connecting said resistance in saidforward path and preventing waves'in said range from affecting saidresistance.

6. The method which comprises producing negative feedback of wantedwaves around a closed loop path containing an amplifying element withthe loop gain for said waves of so much greater order of magnitude thanzero decibels over a frequency band so wide that selfsustaining feedbackof other waves around said path is-produced at a frequency outside ofsaid band-as a consequence of the magnitude of said band width and loopgain and producing a component of loop transmission varying non-linearlywith amplitude of said other waves for maintaining the maximum amplitudeof said other waves below the overload point of the amplifying element.

'7. A wave amplifying device, means for supplying thereto signal wavesof a wide frequency band, a feedback path forming therewith a loop pathhaving the loop gain of so much greater order of magnitude than zerodecibels over a frequency range of said signal waves extending so farupwardly from a frequency at which the loop phase shift is 180 degreesthat self-sustaining feedback of oscillations around said loop pathresults at a frequency above said band,'means in said loop pathselective to frequencies including said oscillation frequency outside ofsaid band and means responsive to the selected waves to produce acomponent of loop propagation varying non-linearly with amplitudevariation of the selected waves for maintaining the amplitude of saidoscillations below the overload value for said device.

8. A wave translating system comprising a source of signal waves of aWide frequency range,

an amplifier connected thereto for amplifying said waves, means formingwith said amplifier a closed feedback loop path producing negativefeedback of said waves around said loop path with the loop gain oversaid range oflarger order of magnitude than zero decibels, said loop,

path having the loop gain decrease so rapidly with change of frequencyat oneside of said range that before the decreasing loop gain reaches avalue less than zero decibels, the'loop phase shift decibels over saidband, said loop gain over said reaches zerodeg'rees and causesself-sustaining .of .said "oscillations, and frequency selective meansresponsive to said oscillations for causing said controllable means tointroduce in a portion of said'loop path a transmission variationnonlinear with respect to the amplitude variation of the selectedoscillations-and thereby limit the amplitude of said oscillations to avalue below the overload value for the amplifier.

9. A wave amplifying device, means for supplying thereto a frequencyband of signal waves, a feedback path for said'device forming therewitha loop path producing negative feedback of said wavesaround said'looppath with the loop gain of greater order of magnitude than zero bandbeing sogreat that at a frequency outside of said'band for which theloop phase shift equals zero the loop gain is at least as great as zerodecibels, frequency selective means responsive to oscillations ofsubstantially said frequency outside of said band in said loop path andmeans responsive to the selected waves to produce a component of looppropagation varying non-linearly with amplitude-variation of theselected waves for limiting the amplitude of said oscillations to -avalue below the overload value for-the ampli- 10. Aowave amplifyingdevice having a feedback path forming therewith a closed loop pathproducing'such'large negative feedback of waves of a given'frequencyband to be amplified by said device that said loop path produces as anincidentof said negative feedback self-sustaining feedback ofoscillations around'said loop path at a plurality of frequenciessimultaneously, said oscillation frequencies being outside of saidfrequency band, means including a plurality of amplitude limitingdevices, one for each of said frequencies, responsive to changeofamplitude of impressed waves to introduce non-linearlyproportionalchange in transmission efficiency in a sportionjof-said looppath for limiting the amplitude of'said oscillationslat each of saidfrequencies tovalues below the. overload value for said device and aplurality of frequency selectors,

one for eachv of said amplitude limiting devices, eachiselectortransmitting to its associated limitingvdevice said oscillations of oneof said frequenciesfto the exclusion of said oscillations of saidotherfrequencies and said Waves.

11. A wave amplifyingdevice having a feedback path .forming therewith aclosed looppath producing negative feedback of waves of a used frequencyband around said loop path, said negative feedbackibeing so great thatsaid loop path produces self's'ustaining feedback of oscillations offrequency outsideof said band around said loop-pathas' an incident ofsaid negative feedback, means in said loop path non-linearly responsiveto impressed Waves to provide amplitude discrimination for limitingtheamplitude of said oscillations; and means. comprising a bandsuppression filter attenuating waves of said'utilized frequencyhand'butfhaving relatively low attenuation for-waves of a wide frequency rangeabove and below said band forexcluding from said limiting means waves ofsaid band.

'- HAROLD S.-BLACK.

